Porteus_Mate-v3.2.2-x86_64: Booted from CD, ran the Porteus Installer, selected the fat32 partition (/dev/sdh1) on my MBR USB stick and selected Install the Bootloader.
Only the /boot and /porteus directories are copied and the stick can only be booted in MBR mode.
I then copied the /EFI directory as well, and now the stick can be booted in UEFI mode as well.
So why is the /EFI directory not copied by the Porteus Installer?

Well, I would not know what the thinking was then as UEFI booting is independent of the USB stick having an MSDOS or a GPT disk label -- as my manual copy of the EFI directory showed.

Anyway, I repeated my test with an GPT USB stick: problem persists.
I then repeated my test with an ext4 partition on the GPT USB disk: problem persists.
I then burnt the Porteus_CINNAMON-v3.2.2-x86-64.iso to a CD and with that repeated my test to the fat32 partition on a GPT USB disk: problem persists.

Hmm, the Porteus Installer (/opt/porteus-scripts/pinstaller) script is not trace (sh -x) friendly. But as far as I can see (at copy_porteus_files() -- lines 629 a.f.) /EFI is never copied ...

The tests until now followed the "From Porteus itself (booted from CD):" path as mentioned in the USB_INSTALLATION.txt file in the root of the ISO.
If instead I choose the "From any Linux Distribution other than Porteus:" path /EFI is copied too and after running the /boot/Porteus-installer-for-Linux.com script the USB stick can be booted in MBR as well as UEFI mode.